The present invention relates, in general, to the prestressing of elongated conduits for conveying hot or cold fluid, and in particular to a new and useful method of manufacturing and prestressing tubular apparatus made of two or more coaxial tubes.
Heavy oil and tar sands represent huge untapped resources of liquid hydrocarbons which will be produced in increasing quantities to help supplement declining production of conventional crude oil. These deposits must, however, be heated to reduce the oil viscosity before it will flow to the producing wells in economical quantities. The dominant method of heating is by injection of surface generated steam in either a continuous (steam flood) or intermittent (steam stimulation or "huff and puff") mode.
When steam is injected down long injection pipes or "strings", a significant amount of thermal energy is lost to the rock overburden (500 to 7000 feet) which covers the oil deposit. In the initial steam injection projects, the price of oil did not justify the prevention of this heat loss, but now with the price of oil at $30.00 or more a barrel, insulation systems for the well injection pipe become economically justified.
Thermally insulated double wall piping structures are known and used, for example, as insulated steam injection tubing in oil wells, or in pipe lines for carrying fluids at elevated temperatures. Such piping is disclosed, for example, in U.S. Pat. No. 3,574,357 to Alexandru et al and U.S. Pat. No. 3,397,745 to Owens et al.
It is common practice for such tubes to be prestressed in order to compensate for differential expansion of the inner and outer coaxial walls or tubes. Such prestressing is done, for example, by elongating the inner tube through such means as heating or mechanically stretching and attaching the outer tube while the inner tube is in such an elongated state. While still held in the elongated state, any heat treatment required for the attachment is completed. However, it is difficult to heat treat the welds while the tubes are under stress. For this reason, it is believed that such heat treatment of the welds is not normally done in the industry, resulting in welds which are more brittle, more damage prone, and more corrosion prone.
After cool down of the heat treatment, if any, the heating or mechanical stretching is then removed and the tubes assume a state of tensile prestress on the inner tube and compressive prestress on the outer tube. While in service, the inner tube becomes hot and expands. This relaxes the tensile prestress before the inner tube goes into compression. In this manner, the inner tube is prevented from buckling.
In an analogous fashion, where the inner tube is adapted to convey cold fluids, the outer tube is heated or mechanically stretched before the inner tube is connected thereto.
Disadvantages of these prior approaches to prestressing double walled tubes or conduits is that the inner, outer, or both tubes must be held in their compressed or stretched state while other manufacturing steps are accomplished such as the connection of the tubes, the heat treatment thereof and the cool-down therefrom.